抄録
We repeated GPS measurements on two twelve observation points in the southern region of Chubu and Kanto district in March 1989 and January 1990 to detect the detailed mode of relative motion between the Philippine Sea and the Eurasian plates. The baseline lengths in this GPS network range from 40 km to 1150 km. In the 1989 campaign the satellite constellation over Japan was very poor and four satellites were visible for only several tens of minutes. The satellite visibility has been greatly improved in the campaign of 1990. The short term repeatability of baseline length is generally less than 0.2 ppm in the 1990 campaign, on the other hand it amounts to 2 ppm in the 1989 campaign because of poor satellite configuration. The strain change derived from GPS baseline measurements, except the lines extending from Hachijyo show the contraction of 0.5 ppm in NWN-SES and the extension of 1.5 ppm in ENE-WSW in the period from March 1989 to January 1990. Although the directions of contraction and extension agree with the result obtained from conventional geodetic survey repeated with 100-year interval, the strain rate does not agree. If we discuss GPS measurement with an accuracy of 0.1 ppm, many error sources have to be taken into consideration. Among them, errors in satellite broadcast ephemerides and satellite configuration are dominant sources. We processed the data by utilizing the broadcast orbits elements. The baseline determinations processed by using broadcast orbits which were obtained in different observation sessions in a day shows systematic differences ranging from 0.1 to 0.5 ppm.
